This invention relates to ignition devices for internal combustion engines, and more particularly to the circuit organization of the power transistor unit for turning on and off the current supplied to the ignition coil.
FIG. 2 is a circuit diagram of a conventional ignition device for an internal combustion engine. In FIG. 2, a control unit 1, including a CPU (not shown) for controlling a transistor 1a, generates an ignition signal in synchrony with the rotation of the internal combustion engine. In response to the ignition signal from the control unit 1, a power transistor unit 2 turns off the current supplied to the ignition coil 3. The power transistor unit 2 includes a filter circuit 4 consisting of a capacitor 5 and a resistor 6, and a pair of transistors 7 and 8 coupled in Darlington connection. The filter circuit 4 reduces the effect of noise on the signal lines, which is induced, for example, by the reflection voltage of the ignition coil 3. Thus, the filter circuit 4 suppresses the interference of the ignition coil 3 on the control unit 1.
The operation of the conventional ignition device of FIG. 2 is as follows. When the CPU turns on the transistor 1a to turn off the transistors 7 and 8, the current flowing through the primary side of the ignition coil 3 is interrupted and thus a high voltage is induced at the secondary side of the ignition coil 3. The high voltage thus induced at the secondary side of the ignition coil 3 is supplied to the ignition plug (not shown) of the internal combustion engine.
The above conventional ignition device has the following disadvantage. The power transistor unit 2 includes, in addition to the transistors 7 and 8, the filter circuit 4 consisting of the capacitor 5 and the resistor 6. Thus, the number of necessary parts are multiplied and the structure is thereby complicated. This increases the production cost of the device.